The present invention relates to an aluminium alloy for components having increased strength with a yield point Rp0.2>120 MPa and at the same time a high elongation at break A>7% in the cast state, a yield point Rp0.2>200 MPa and at the same time an elongation at break A>6% after a T5 heat treatment or a yield point Rp0.2>200 MPa and at the same time an elongation at break A>9% after a T6 heat treatment, in particular for structural parts and chassis parts of a motor vehicle.
Good flow and mould-filling properties and solidification characteristics are critical for structural components produced by a pressure casting process, in particular thin-wall components, and also when the pressure casting process is used for chassis parts. Thin-wall structural components are of particular interest in the automobile industry since these provide a weight advantage for the same component function as a result of lower materials usage, and this in turn reduces the operating costs and decreases environmental pollution.
The pressure casting technique now allows complicated components having a high strength and high elongation to be produced. Chassis parts are customarily manufactured in many places by other casting processes such as chill casting. The reason is that these components produced by the pressure casting process do not achieve the required strengths or do not achieve them at a satisfactory elongation in order to ensure reliable operation.
To achieve the required mechanical properties, especially a high ductility, a heat treatment, for example according to T6 (solution heat treated, quenched and aged hot) or T7 (solution heat treated, quenched and overaged), is usually carried out in the case of structural and chassis parts made of pressure casting alloys of the AlSi10MnMg type. This changes the cast microstructure of any component which then satisfies more demanding requirements in respect of strength and elongation at break. While an alloy of this type in the cast state has a yield point Rp0.2 of about 110 MPa at an elongation at break A of 4-5%, an increase to above 150 MPa at not less than 7% elongation can be achieved by means of a T6 heat treatment. This is based on the strengthening effect of precipitation hardening in which the alloying elements Mg and Si participate. In addition, coalescence of the Si eutectic increases the ductility. Such a heat treatment is, for example, carried out as follows: a solution heat treatment in the temperature range from 450 to 535° C. is followed by quenching in water or in air to temperatures below about 100° C. As a result of the solution heat treatment, the alloying elements are homogeneously finely distributed due to diffusion processes and constrained in the α-Al by the quenching. In addition, the Si eutectic is spheroidized. The alloy now has a high ductility but only a low strength. As a result of the subsequent hot ageing at 150-250° C., fine uniformly distributed Mg2Si precipitates are formed and these in turn increase the strength of the material. Depending on the temperature profile of the T6 heat treatment, the mechanical properties can be optimized in terms of either strength or elongation at break, by which means a very wide property and thus product folio can be obtained from one alloy. To reduce production costs, a T5 heat treatment, i.e. hot ageing at 150-250° C. without prior solution heat treatment, can also suffice. Here too, the strength increase is due to formation of Mg2Si precipitates, but to a lesser extent since the quenching effect of a component taken from the casting tool is less pronounced and the proportion of magnesium forced to dissolve in the α-Al therefore also decreases.
Far higher strengths of up to 600 MPa for the yield point Rp0.2 are achieved by mechanically alloyed AlZnMg and AlMgCu alloys because of their greater hardening potential. In these types of alloy, the strengthening effect is based on the precipitation hardening of the alloying elements Mg, Cu and Zn (W. Hufnagel et al., “Aluminium-Taschenbuch 14th edition”, Aluminium-Verlag Düsseldorf, 1988, p. 46ff). However, owing to their susceptibility to hot cracks and their tendency to stick in the casting mould, these alloys are not suitable for pressure casting.
As further demands made of a structural or chassis part produced by a pressure casting process, mention may be made of, in addition to the demanding requirements in terms of strength and elongation, corrosion resistance, suitability for welding and life of the casting moulds. A further requirement is the dimensional stability of the components after heat treatment in order to be able to ensure problem-free assembly of the vehicle body.
Complicated solution heat treatments have, apart from additional economic costs for the heat treatment itself, the disadvantage that components tend to distort as a result of the sharp quenching, which can lead to further machining work and an increased reject rate.
It is an object of the invention to provide an aluminium pressure casting alloy which makes it possible, due to increased strength combined with high elongation, to make both structural and chassis components in a pressure casting process. This preferably includes chassis parts which, owing to the demanding mechanical requirements (e.g. yield point Rp0.2>200 MPa at an elongation at break of A>6%) and the component geometry, tend to be produced by processes other than the pressure casting process. In addition, it is an object of the invention to ensure good castability and filling of the mould. Furthermore, the alloy should allow very many joining techniques, have high dimensional stability and have good corrosion resistance.